The present invention relates to a nuclear magnetic resonance imaging system (also referred to as MRI system in abbreviation) for obtaining a topographic image by making use of a nuclear magnetic resonance phenomenon (also referred to as NMR phenomenon). More particularly, the present invention is concerned with a method and an apparatus for measuring an NMR signal while compensating for nonuniformity of a static magnetic field.
In recent years, the MRI system attracts a good deal of attention and is coming into wide use because the system can make available not only anatomical information such as an image reconstructed by visualizing X-ray absorption coefficients obtained by an X-ray CT (computerized tomography) system but also biochemical, chemical shift and bloodstream information.
However, the MRI system suffers some problems in connection with NMR measurement. As one of such problems, there can be mentioned nonuniformity of the static magnetic field.
For example, in the case of an MRI system of permanent magnet type in which permanent magnets of Nd- Fe-B series are used, the temperature coefficient of one magnet element is as great as -900 ppm/.degree.C., which in turn means that uniformity of the magnetic field varies remarkably as a function of temperature. However, when the magnetic circuit is implemented in a self-shield structure, a circumscribing yoke member exhibits a large thermal time constant, whereby the abovementioned value of the temperature coefficient can be made smaller. In practical application, a thermoregulating system is provided in an effort to ensure stability of the whole magnetic circuit over a time period to thereby prevent nonuniformity from taking place in the static magnetic field.
In the MRI system in which a vertical magnetic field structure is adopted, magnetic poles of N and S polarities are disposed vertically in opposition to each other, wherein thermoregulation is performed for the top and bottom magnets. In this conjunction, it is noted that when the thermoregulating control of the top and bottom magnets undergoes deviations for some reason, nonuniformity components make appearance in the static magnetic field, wherein a linear nonuniformity component varying linearly along the Z-axis taken in the vertical direction becomes more noticeable to thereby exert adverse influence to the image reconstructed.
In general, in the measurement of the NMR signal in the MRI system, a signal having a maximum amplitude is obtained at a time point when a phase encoding quantity is zero, as shown in FIG. 1 of the accompanying drawings. Further, as can be seen in FIG. 2 which illustrates schematically the data obtained by an NMR signal measurement, a signal peak is located at the center A of a field of view (also referred to as visual field) extending along a phase encoding direction and a frequency encoding direction (read-out direction), wherein NMR signal components of same levels are distributed coaxially about the peak A. Under the circumstance, in the MRI system known heretofore, dynamic range of an analogue-to-digital or A/D converter is set up with the value of the NMR signal corresponding to zero phase encoding being selected as a maximum value of the dynamic range, whereon the image processing is performed. However, when nonuniformity takes place in the static magnetic field, giving birth to a nonuniformity component Gyo in the static magnetic field at zero phase encoding which component is equivalent to a phase encoding gradient magnetic field and has a minus or negative sign as viewed in the phase encoding direction, then the signal peak as measured actually is shifted from the point A to a point B shown in FIG. 2. Parenthetically, FIG. 3 of the accompanying drawings shows a sectional view taken along a line II--II in FIG. 2. Referring to FIG. 3, when the value of the signal at zero phase encoding is recognized as the peak, as with the case of the prior art system, then the signal peak value which is intrinsically 1.0 is recognized to be 0.5, which means that a valid signal component is deleted, bringing about distortion in the image reconstructed.
Further, when NMR measurement is performed by resorting to a gradient echo method and a spin echo method, a slice selecting gradient magnetic field is applied for selecting a slice at the time when an object under inspection is irradiated with radio frequency (RF) pulses. In this conjunction, if a gradient magnetic field ascribable to nonuniformity of the static magnetic field, the thickness of the slice as well as position selected for the slice undergo variation or degradation in respect to the accuracy, giving rise to another problem.
In the MRI system, occurrence of abnormality in the thermoregulating system can not endanger the safety. However, when nonuniformity takes place in the static magnetic field due to the abnormality, there will occur image distortion as well as variations in the slice thickness characteristics, as pointed out above, which of course provides difficulty for diagnosis, a problem to be solved.